Bug#659644: Increase minima and maxima in CMA

[Jacob Nevins]

Since gna.org has gone away:

In 2.6, the Gtk3 client (intended to be the default client in that
version) has gained options to change the limits. (On reflection, not
sure why we made this client-specific.)

Original Gna ticket (archived):
<https://web.archive.org/web/https://gna.org/bugs/?16184>
Gna ticket where this was added (archived):
<https://web.archive.org/web/https://gna.org/patch/?6352>
Upstream commit:
<https://github.com/freeciv/freeciv/commit/5c31635c4a03674a198e7bfa35dbf56e6e4a0291>